The advance in structural studies of biological electron transfer (ET) presents the exciting opportunity of progressing towards a more molecular level in correlaring structure and function of electron transport proteins. The overall aim of this project is to advance the understanding of ET processes in biological and chemical systems to a more niicroscopie level using the available structural information and computer simulation approaches. During the past grant periods we developed, refined and examined a wide range of strategies for studies of ET processes. These approaches were used in studies of the energetics and dynamics of the primary event in bacterial RC, in evaluating redox potentials of proteins, and in studies of other aspects of ET in proteins. Although our studies were very useful they did not resolve all of the fundamental problems in the field and actually in some cases they presented new and exciting questions. In order to fiirther progress in this important field we propose the following projects: (i) We will conduct a systematic evaluation of the redox potential of proteins, focusing on iron sulfiir proteins. We will concentrate on all-atom models trying to understand the role of water penetration and the effect of averaging over differet configurations with different degrees of penetation. We will use our understanding in refining computationally efficient models with simplified solvent representation. (ii) We will attack the nalin open questions about the primary ET in bacterial RC using microscopic simulations. This will include (a) exaznination of the time dependence of the dielectric screening, (b)analyzing the local dielectric constant in the sites of the electron- transporting chromophores and relating it to a recent interpretation of Stark effects, (c) determining the energetics of the charge transfer-states of the special pair, (d) studying the energetics of mutatin~charged residues and (e) exploring the origin of the oscillations in the time dependent spectra of P*. (iii) We will study the reorganization energies of biological donor-acceptor complexes. (iv) We will simulate ET reactions in solution focusing on the following: (a) evaluating reorganization energies of covalently linked donor-acceptor pairs, (b) studying the effect of solvent dynamics while representing the solute by consistent quantum mechanical approaches, (c) developing density matrix methods, and (d) pushing the frontiers of calculations of electronic coupling matrix element by treating the solvent quantum mechanically.